Electro-optical system



Feb. 25, 1958 K. D. sMn-H 2,824,975

ELECTRO-OPTICAL SYSTEM ATTORNEY 2 Sheets-Shea# 2 F/G.8

ELECTRO-OPTICAL SYSTEM &

Feb. 25, 195s Filed April 5, 1944 /N VE N 7 0R ,a KasM/TH BV :Jwgf /MATTORNEY l EQU/VALENT LIGHT M/CROAMPERES United States PatentLECTRO-OPTICAL SYSTEM Kenneth D. Smith, White Plains, N. Y., assigner toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application April 5, 1944, Serial No. 529,659

19 Claims. (Cl. Z50-214) This invention relates to transducer systemsand more particularly to electro-optical systems.

An object of the invention is to provide an improved transducer system,for example an electro-optical system, having a nearly constantpercentage sensitivity, that is, a system in which the output voltage isnearly proportional to the percentage modulation of a modulated staticcondition but is independent of the magnitude of the static condition.

In order to attain this object, one or more non-linear resistiveelements in a series circuit with a light sensitive electric device areused to control an amplier which is variably biased in accordance withthe current in the series circuit under diierent static lightconditions. In an example of practice of this invention a vacuum tubeamplifier which is condenser-coupled across one or more of the elementsof a circuit comprising a photoelectric cell, a source of direct currentand a varistor connected in series is biased by means of a highresistance connected in shunt to the coupling condenser. The bias of theamplifier in this arrangement is dependent upon the current in theseries circuit under diierent static light conditions.

Subject-matter divided from this application is disclosed and claimed inapplication Serial No. 608,702, filed August 3, 1945, forElectro-Optical System.

Other illustrative embodiments of this invention will be describedhereinafter.

A varistor, as the term is used herein, is anon-linear resistance.device which may assume aV variety of forms but is always a conductiveor semi-conductive device, never an electronV emissive device. Varistorelements suitable for use in embodiments of this invention are discs ofav semi-ceramic material composed of siliconA carbide, clayv and carbonpressed and tired. A characteristic of such a device is that theresistance is a nonlinear function of the current flowing in the device.The impedance at any given voltage is inversely proportional to the areaof the disc. Therefore, almost any desired operating impedance may berealized by a choice of larea of the varistor disc. This type ofvaristor, its composition and method of manufacture are described in Me-Eachron Patent 1,822,742, issued September 8,v 1931.

One use to which this invention is adapted is to detonate the explosivecharge of a projectile as it moves into proximity to a target due to areduction inthe illumination of the photoelectric cell. The normal orstatic illumination may be large or small, but for any given percentagereduction inthe illumination the sensitivity remains practicallyconstant. Thus, as the projectile moves past the target, which may be anairplane, a certain percentage of the illumination of' the photoelectriccell isv cut olf at a given rate dependent upon the speed of theprojectile and the nature of the airplane. However, the sensitivity ofthe detonating unit remains reasonably constant, no matter what theabsolute intensity of the illu- ,muationmay be;

Another use for this invention is in the measurement of percentagemodulation of a light beam for a Wide range of valuesv of static light.In any arrangementV where the. change in output voltage is constant fora given percentage reduction in light anywhere within the working rangeof light variation, that arrangement may be used to measure percentagemodulation within that light range by utilizing a peak-to-peak meter`energized by the alternating current component of the voltage producedby energizing the photoelectric cell with modulated light and calibratedin terms of percentage modulation. For light modulated at any constantpercentage theV reading of the meter would remain constant for allaverage values of light Within the operating rangeV of light values.However, the meter reading would change if the percentage modulation ofthe light were changed.

A similar arrangement comprising a peak-to-peak meter may be utilized toindicate how nearly various arrangements of photoelectric cells,varistors and biasing resistors produce voltages which are proportionalto the percentage modulation of the static light at various values ofstatic light. For this purpose the percentage modulation is keptconstant While the average value of the light is changed, changes inmeter reading indicating departure from constant voltage response.

For best results, the peak-to-peak meter should be a; vacuum tube meterwhich indicates peak-to-peak values; of the impressed voltage for anywave shape or" the im-V pressed voltage. A cathode ray oscilloscopeproperly` calibratedv may be used as thel indicating instrument.. Onesource of modulated light is an ordinary iilam'entary' electric lampenergized by Gil-cycle alternating current.A The percentage modulationof such alight may be varied by energizing the lamp with, superimposedalternating current and direct current and varying the relative voltagesof each. Many other sources of modulated light are known. v

This invention will now be described more in detail having reference tothe accompanying drawings:

Fig. l is` a circuit embodying the invention in which the controlelectrode of a pentode amplifier is variably' biased by the staticvoltage across' a varistor by means of a high resistance connectedbetween the grid and' one terminal of the varistor in shunt of thecoupling condenser;

Figs. 2 to 9 are modified circuits embodying the invention;

Fig. l0 is a diagrammatic illustration of an arrangement for determiningthe modulation of a source of light or the effectiveness of the circuitsof Figs. l to 9;

`Fig. 1l illustrates other load circuits suitable for use with'thecircuits of Figs. l to 9; and

Fig. 12 shows the relationship between current in theV photoelectriccell andl relative sensitivity, that is, the relative change in loadvvoltage for the'V same percentage change in illumination of thephotoelectric cell in arrangements'according to Figs. 8 and 9.

The same reference charactersare used to indicate identical elements inthe several iigures of the drawings;

Referring now to Fig. 1, a photoelectric cell 10, a varistor- 11, abattery 12 and a protective resistor 13are connected'together to form aclosed series circuit. Coupled; across the varistor 11l is an VampliiierstageVl to which amplifier stage is connected aload circuit L. Theampliier V1 comprises a pentode vacuum tube 14; the

control grid of which is connected through a condenser' i 15 to terminal16 of varistor 11. The cathode of pentode 14 is connected to the otherterminal 17 of varistor 11. Connected between the gridand cathode ofpentode 14* is a grid leak resistor 18.` Iii-order to provideaVariable'` bias voltage for the control grid of pentode 14 according.`

to this invention, a resistor'19 of relatively high resistance isconnectedbetweenA the controlfgridY of pentode 14Y and j the terminal 16of varistor`11. This bias voltage is pro- 7 portional tothe voltage dropacross varistor 11 which inV turn is a 'non-linear function of theillumination of the Yphotoelectric cell Y10. The screen grid of pentode14 is energized Aby Vbattery 20 through resistor'21. The loadV circuit Lis connected between vthe plate and cathodeof pentode 14.

. yIt the voltage versus currentI characteristic of varistor 11 wereexactly logarithmic and the amplilier stage V1 Y' Y' Wereexactly linearover the working-range of illumination ofthe cell lib-theresponseobtaine'd in the load circuit L of resistor 19 according to thisinvention withY a varistor 11 and vacuum tube 14 which in themselves donot give the desired response.

In the circuitof Fig. 1, it

photoelectric cell and varistor series circuit byrtwo paths, namely, analternating current path through condenser and a direct current pathVthrough resistor 19. Such coupling ischaracteristic of this invention.VThe voltage supplied through" the direct current path changes theVirnpedance of the'spaceV path between the control grid and cathode, atleast for some light values within the working range, so that thevariation inloutput current or voltage Vis :substantially constant forany given percentage change Vin illumination within the workingrange oflight values.

However the biasing voltage supplied through the direct.V

current path may be used to modify the response in any othermannerdesired. Y Y Y 1 In the arrangement of Fig. .2 a resistor is connectedin series with the varistor'lland the voltage drop across Y Yresistor 25is impressed through grid leak resistor 18 on the grid of pentode 14.The Vvoltage drop Vacross resistor 25 is proportional to theillumination of photoelectricj cell 10. The resistance of yresistor 25is so chosen that YYin cooperation with the variable impedance of thecontrol grid to cathode space of pentode 14 it will compensate for thedeparture of the voltage dropacross the varistor 11 from a logarithmicvariationrwith respectY to photoelectric'currentV or illumination.

In the arrangement Vof Fig. 3 Vthe series circuitlcomf Y prisingphotoelectriccell 10, varistor llrand battery 12, Vincludes additionalresistors 30 and 31 and a biasing bat.

tery 32. The control grid of pentode 14 is connected 10,` the condenser15-constituting an alternating current path between the photoelectriccell and varistor series circuit and the amplifier V1. vTheresistor 31functions both asa grid leak for pentode 14 and as a source 'of nvariable biasing voltage by reason of the direct conductive; connection34 Ybetween Yterminal 35 of resistor Y31V and the controlA Vgrid ofpentode14. YAY directY conductivek s connection 36 connects the cathodeof pentode 174 to theV 'Y terminal 37 of Vbatteryf32. The connection 34fromj-theV grid of pentode 14 to the terminal V35'is va direct curi-entpath'from amplifier V1 to the photoelectric c'ell and Varis- Y Ytor'series circuit. YThe' voltage dropacross photoelectric cell10 andresistor 30 is impressedV on Vampliiier V1.

YThe resistor modifies the volt through condenser 15.

However,

` andso changes the impedance Yin accordance with the` Y should be notedthat the input circuit of Vthe ampliiier stage V1 is coupled to -thethrough condenser 15 to terminal 330i photoelectric cellV s i Vageimpressed on amplifier V1 byzway of Vthe alternatingY currentpath-through condenser 15.V VBy properly "selectaing'rthe'circuitVelements injFig. 3 and desired're'lationship j Y vmay be Yobtainedbetween light on the photoelectric cell Y 10V andcurrent in theloadcircuit'L.V ln this arrangement Y the biasingfb'attery 32 laidsvthebattery 12 in energizing. the series circuit comprising thephotoelectric Vcell'l() and T5 is inserted in the series circuitbetween'the terminal v41 of varistor 11 and the negative Vte'rrr1inal42of battery 12.

The control grid of pentode'14 is connected through grid leak resistor18 to the terminalA 42 so that the voltage. drop across resistor 40places a variable negative bias on the control grid of pentode Y14 withrespect to the cathode of pentode 14; Y

The arrangement voltage makes the'grid positive Withrespect to thcathode current through the photoelectric cell 10.

lnrthe arrangement of Fig.VV 6 the voltage drop across to-terminal 51 ofresistor 50by conductor 52. .Thecathode of pentode 14 is connected tothe terminal 5730151 resistor 5t).V In this arrangementthe voltage drop.across the photoelectric cell 10 is impressed on the amphrer V1 throughthe alternating current path by way of condenser.. the directY 15 andthe Variable biasingpot'en'tial through current path byl way ofconductor V52.'

The arrangementof Fig. 7islike'that In the arrangements Vof Figs.

for'the controlrgrid of pentode '14 may bejvaristorsunder someconditions instead of hired-resistors.. The response Y, to be attainedwill determine the kind ofvaristor to be` used. Y f In the arrangementof` Fig.

being impressed through condenser/15 on the control grid and insertingthe'protective resistor'13.

An arrangement for utilizing the circuits ofFig. 5 isrfruuch likethatr'of Fig. 2 I except that'the photoelectri'c" cell Y10 and thevaristorrll';Y Y have been interchanged. 'VTlie biasingrv'oltagerrfortheV of pentode 14 is supplied by resistor 45. This biasing c of Eig. V5Yexcept Y that abiasing batterySS isconnectedbetweenthe resistorY 45 andthe'battery 12. Battery 55 yaidsthebattery12VV with respect Yto theseries circuit vincluding thelrphotof"V electric'c`ell"10 aridV varistory11 but opposes the-Voltage Y drop across resistor 45injdeterminingjthebiason the,` v control grid Yof pentode 14. Y The battery 55j andjresistor 45 may be sochosenj as tocause grid current to flow for' vlight values within only a portion ofthe range of work? Ving lightvalues torcontrol the impedance of the'grid toi cathode path ofthepentode1'4.` Y Y Y v 1 to.7, the'rersi'stors'19, Y 25, 30, 31, 40, 45and 50 which provide a variable bias 8; Va second varistorg60 f shuntedby iixed resistor 61 is used to furnish Vthe variable biasing voltagefor the control grid of pentode14.`l Af iixed resistor 62' is connectedinV serieswitlr varistor'11,VK the total voltage dropacross varistor 11andk resistor 62'Y u of Figs; l'to9 Yis illustrated in Fig. 10. Box 65and'photoelectric cell'lllf` represent an arrangement like any one ofthose illustratedV Y in Figs. l tov 9 to the `leftV of theV line X-X; Anaudio 1 ampliiier 66 is connected as loadin place Yof Ythe'load L tothel right of liney XJ-Xy inFigsril to 9.x' pYeak-to-V j f peakYvoltme'ter 67VV the readings of Vwhich are indicated 'd 'on an ammeter Aor a cathode *rayY oscillograph 68 may i Y be connected alternativelytofthe Voutput `ofarnpliiier 66 i by switch V69.V A second'photoelectriccell 70 is connected in serieswith abattery71fY and resistor 72 toafrnicroanl-V `V f meter 73 the'reading ofA which gives a-me'asure.off-the;Y n.

average VVillumination of'photoelectriccell 70. I' Photoelec-l triccells 10`and Y7 0 are equally illuminated YbyY light from f v,anincandescent ilament la1r1pf74V which may'be1posif'f tioned atdiie'rent distances frornftheV cells-10=and V70.; v. as indicatedVby-the double-headed arrows- 775..; Y,The iilment may be energized by.60 cycles alternating'eurrent-or c by 60 cycles alternating VcurrentYsuperposedQl-on '.'direc'tl'lf f asaagsrs` ga 3 current. The lightproduced by either type of energization is a modulated light, that is, alight of some average value varying cyclically in accordance with thewave shape of the energizing current.

Thevarrangement of Fig. l may be used in two ways, namely, rst todetermine the characteristic `of the circuit represented by block 65 andsecond, to determine the percentage modulation of the light from lamp74. When the circuit represented by the block 65 has such acharacteristic that for equal percentage changes in the illumination ofphotoelectric cell equal changes in output voltage result as indicatedby meter A, assuming, of course, that the illumination falls within theworking range of the illuminations of the arrangement, the meter A maybe calibrated in terms or" percentagemodulation of the light and willshow Athe percentage modulation of light from any source of lightwhether it be lamp 74 or some other source such as a lamp used forlighting a room or even daylight.

The graphs of'Fig. 12 show the characteristics ofthe circuits of Figs. 8and 9 for a given percentage modulation of the light as determined bythe arrangement of Fig. 10. The illumination expressed in microamperesare the abscissas, being the readings of meter 73 for various positionsof the lamp 74 and the numbers representing the relative sensitivity arethe ordinates, being the readings of the meter A of peak-to-peakvoltmeter 67 for corresponding positions of lamp 74. Graph 80 shows thecharacteristic sensitivity'of the arrangement of Fig. 8 using a certainpair of varistors 11 and 60. Graph 81 shows the characteristicsensitivity of the arrangement of Fig. 9 using the same pair ofvaristors 11 and 60 as for the graph S0. 1iesistor 62 has been removedand the protective resistor 13 has been inserted, reducing the relativesensitivity somewhat but improving the ilatness of the 'n'characteristic. Graph 82 also shows the characteristic sensitivity ofthe arrangement of Fig. 9 using a higher impedance element for varistor11, the other elements being the same as `for graph 81. The sensitivityfor all the arrangements represented in Fig. 12 falls o at very lowilluminations or current values. The sensitivity of the circuit of Fig.9 also falls rapidly at very high illuminations or current values due t0the protective fixed resistor 13. The data for the graphs of Fig. 12were all obtained for the same percentage modulation of the light fromsource 74.V

The circuit of Fig. 1l is another load circuit which may be substitutedfor the load L in any of the arrangements of Figs. 1 to 9. It comprisesan amplifier stage V2 adapted to be connected to amplifier stage V1 ofany of the arrangements of Figs. 1 to 9 and a thyratron stage T coupledto the'output circuit of amplifier stage V2. The output circuit ofamplifier stage V1 is coupled to the input circuit of amplifier stage V2by resistor 85, series condenser 56, series grid resistor 37, grid leakresistors 88 and 89, shunting condenser 90 for resistor 89 and shuntgrid condenser 91. The output circuit of ampliiier stage V2 is coupledto the input circuit of thyratron stage T by resistors 92 and 93,condenser 94 shunting resistor 93 and battery 95, series condenser 96,grid leak resistor 97 and negative biasing grid battery 98. The outputcircuit of thyratron stage T comprises load resistor 99 in series withammeter tl and battery 9S. Ampliiier stage V2 includes a pentode vacuumtube 1131 and thyratron stage T, a thyratron tube 102. The cathodes oftubes 14 and 1191 and thyratron tube 102 are heated by current from abattery, not shown. The screen grid of pentode 101 is energized frombattery 95 through resistor 103.

The portion of Fig. ll to the right of line X-X may be connected to theportion to the left of line X-X in any or" Figs. 1 to 9 to form varioususeful arrangements. Also in Fig. l1 the portion to the right of lineY-Y may be connected to the portion to the left of line X-X in any ofFigs. 1 to 9 to form other useful arrangements. For example, if theportion of Fig. l1 to the right of line the portion to the left of theline X-.X of Fig. l, an arrangement results in which an impulsiveincrease in the illumination of photoelectric cell 10 is necessary tolire the thyratron.

As an aid in practicing this invention certain construction data willnow be given. This data is merely illustrative since the invention maybe embodied in many forms diiering widely from that given herein. In thearrangement of Fig. 9, for example, from which .the data for graph 81 ofFig. 12 was obtained, certain elements are as follows:

Photoeiectric cell 10 RCA C-7052 phototube. Resistor 113 3 rnegohms.

Resistor 18 100 megohms.

Resistor 61 4 megohms.

Vacuum tube 14 Hytron HY145X'I`. Condenser 15 0.0005 microfarad. Battery12 90 volts.

Varistor 11 is a non-linear resistor element made up of silicon carbide,clay and graphite after the manner disclosed in McEachron Patent1,822,742. The ingredients are granulated and thoroughly mixed in theproportions of 60 parts of silicon carbide, 40 parts of clay, and 1 partof graphite. After wetting with water andbeing partially dried thismaterial is pressed into discs and fired. Each disc has an outsidediameter of 5%: of an inch, a thickness of .030 of an inch, and acentral hole 9/32 of an inch in diameter. The faces only of each discare coated with tin with a metalizing gun. Each completed disc has sucha resistance that 20 volts applied between the tin coated faces producecurrent of between 10 andl 3.3 milliamperes, and 10 volts applied in thesame manner produce not less than 0.21 milliampere. This varistor isformed by dividing one disc into ten equal sectors and clamping tivesectors face to face between copperelectrodes. The resistance of thisunit at one microarnpere is 22 megohms.

Varistor 60 is similar to van'stor 11 except that the area of each ofthe rive sectors is approximately twice that of the sectors comprisingvaristor 1i.

What is claimed is:

1. An electro-optical System comprising a primary series circuitincluding a light sensitiveelectric device and a resistor, theresistance of said resistor changing in response to changes ofillumination of said device, a vacuum tube amplier including an inputcircuit and an output circuit, a coupling for alternating currentcomponents only between said series circuit and said input circuit, aconductive coupling including an impedance also between said seriescircuit andv said input circuit, and a load circuit connected to saidoutput circuit.

2'. An electro-optical system including a primary closed series circuitcomprising a light sensitive electric device,

a varistor and a sourceo'f direct current, an amplifyingv circuitcomprising an element'the bias of which changes the amplifying ability'of said amplifying circuit, a coupling circuit including a condenser forcoupling an element of said primary circuit to said amplifier, and ahigh resistance connected across said condenser to change the bias onsaid amplier when the static light condition changes.

3. A primary closed series Vcircuit comprising a photoelectric cell, avaristor and a source of direct current, a vacuum tube amplifiercomprising a cathode, an anode and a control electrode, a highresistance connectedbeconnection 1 from .one terminal .ofiY saidvaristor to said Y -cathodega condenser connected between theV otherter/Ini-A nal'of said vvaristorrand Vsaidrco'ntrol electrode, aresistance i source of direct'current, said resistor being connected toone terminal of one vof said varistors and a second varistor beingconnected to the other terminal of said one Varistor,

Va'vacuum tube amplifiercomprising a cathode, an anode 1 and a controlelectrode; a condenser connected from the terminal of said resistor.remote from. said one varistor Y to Vsaid controlgelectrode, a highresistance connected Y between the joinedy terminals of saidtwovaristors and t rsaidcontrol electrode, a conductive connection fromthe other"terminalV of said second varistor to said cathode, Y. :and aload circuit connected to saidranode and cathode.`

6.- A'primary closed series circuit comprising aphoto- 'eelcctricfcelh.a Yplurality VVof varistors, a resistor and a -sourceof .directcurrent, said resistor being connected to one terminal of one of saidvaristors anda second varistor being connected to the Yother terminal ofsaid one varistor, 'a vacuum tube amplifier comprising a cathode, ananode 'and a control electrode, a -condenserfconnected from the terminalof Vsaid resistor Vremote from said one varistor to 'said controlelectrodef'a high resistance. connected between the joined terminals ofsaid two varistors. and

said .control electrode, a conductive connection from the other.terminal ofV said second varistor to said cathode,

Ya second'resistor connected in shunt of the terminals of said secondvaristor, and a load circuit connectedto said anode Yand cathode. 'Y t.Y

7. A primary closed series circuit comprising a photoelectric cell, VaVplurality of viaristors and a source of direct current, one terminal ofoneV of said varistors being cony nected to one terminal of a secondof'said varistors, a vacuum tube Yamplifier comprising acathode, ananode and a control electrode, Va condenser connected between theterminal of said one varis'tor remotefrom said second l current, oneterminalof'one'of said varistors being con- Y electric cell, aplurality. of varistors and a source of'directV i nected to one terminalof a second of said varistors, a` i Vvacuur1ltulie ar'npliterkcomprising a cathode, an 'anode .anda control; electrode, a condenserconnectedbetweenV the V,terminal of said one varistor remotefrom saidsee-f f Vond. varistor and saidcontrolelectrode, a'highV resistanceconnected between the other terminal of said one varistor renttherefore, a vacuumY tube amplifier havingi.;` cat hf-r esageran Yseriesfcircuit comprising a lightlsensitive electric device,'a varistorand a'source of Vdirect current, an"`amplifying. Vcircuit comprising abias element, the bias of which respectl to another [element of-saidVamplifying 'circuit' ductivecoupling also betweenV saidseriescircuit'and said I input circuit, anda load circuit connected tosaid output circuit; j Y

vill.,Arrfelectrooptical'Ysystem comprisingra pi'irnarylrV seriesV`circuit including Va light-sensitiveelectric device and a resistor,the. resistance of said 'resistor'Y changing in .P

response to changes of illumination-of said` device,-a'va'cu-Y iurrrtube amplifierv includinganv input circuit. and an output circuit, acoupling fork alternating current cmponents l.' only ,between saidseries circuit andsaidfinpuh circuit, Y ai conductive couplingalsobetween said'seriesv circuit f and JVsaid' input circuit, meansltoYilluminatesaidjliglit-j sensitive electricdevice withV modulatedlight'randY means L connected Yin shuntof said condenser, and a loadcircuit 5 changestheV amplifying ability-of said amplifying circuit,. Yi connected to saidanode and cathode.- Y al circuit lloi"V `alternating'current Y' components onlyL 4: lA primary closed series circuitcomprising a'photocoupling.saidamplifyingcircuit to said primarycircuit, Y electric-cell, a varistorand'a first resistor withthevaristor a1 conductiye connection between said other elementof connectedbetw'eemthe photoelectric cell and the resistor, said amplifyingcircuitanda lfirst point of said'primary i afvacuum Vtube,-Vamplifier'comprising a cathode, ananode V10 circuit, a second conductiveconnection between said 'bias a-control electrode, a condenserVconnected between 'element and :aY second point,V ofVA said Aprimarycircuit Y 'saidcontrol .electrode and the terminal of said Varistorwhich is always positive .withV respect .to said first point. Y' nearerto said photoelectric cell, a high resistance resistor 1.10. Atransducer system, including aY primary electricA connected between saidcontrollelectrode and the other' circuit comprising, in a closed seriescircuit,-a current' Y p terminal of said varistor, a conductiveconnection between 15 controllerv and a'resistor, the resistance of saidresistorV y Y the terminalof said rst resistor remote from said-varistorchanging `in response` to a change of current therein prov- .V i andYsaid cathode,`and a load circuit connected toY said duced by the"action ofsidcurrent controllerQanampli-g anode and cathode. Y Y tyingdevice 'having an input circuit'and .an outputV cirff 5. A primaryclosed series circuit comprising a photocuit, a coupling for alternatingcurrent components only electric cell, a plurality of Varistors, aresistor and a 20 betweenY said series circuit andsaidinp'ut circuit, a'con-r including a'pueak-topeak meter connectedntos'aid output-i Ycircuit;

`712Anlelectroioptical system Vcomprising a photoe1ectric cell, a vacuumtube amplifier having a cathode, 'an anode'and a control electrode, aninput circuit for, said amplhierincluding said'cathode and ,controlelectrode,

anioutput circuit for saidV amplifier including said cathode. i

and anode, Va'hcoupling Vcircuit for alternating currerltfcomffY ponentsonly.V connected between Vsaid photoelectric'cell and saidinput circuitincluding a varistor, means to change f the direct current bias `onVsaid control electrode with respect to said 'cathode in accordancewithj changesiinr,l

thervoltage acrosssaid varistorresulting rfrom changesV in'gtheillumination of said photoielectriccell, anda loadVVV Vcircuit connectedto the output circuit .ofr'said ampli-I 'l andanodcg'acoupling'circuit'foralternatingcurreiit'com` Y* ponente onlyconnecte'dbetween said( photoelectrocell and said inputcircuitVincludingaavaristor, means tof:k

. K13].,Anelectroopticalfsystem 'comprisngfa photoelec.-`

change the directV current Vbias Y,on saidcontrol electrode." lwith'respectto'said cathodelin accordanc'ewitl'i.changesA invtliejvoltagef across said varistor .resulting Vfrorrrcl'ian'gesY f Vinthe'illumiiation of saidfphotoelectric cell andfinclud-fj ingfalealagepath of constant high r'eSStanCeomy said Y control Velect'ro'de'to saidcathode,fand. a loa'dlcirc'uit confr i nected to th'eoutpntvcircuitofsaid amplifier. Y

, I4, Ari'electro-optical` system fcornprisingaseries path j ode,ananode and a grid, a condenser having Aoneternii- .Y 'Y load. circuitconnected to .said anode and cathode. 'Y 9549.electro-Optical;'systemincludinga pnniarycloseu 7 andsaid control electrode,'a'conductiveconnectionbe nalcolllleCdV O .Said .grid and -thlothlermlll COU-fi* 'YVt\.'yee`i'i"tlie .terminal of said second varistor remote Vfrom l saidAonefilaristorfand said cathode,-aresistorconnected v in shuntot theterminals'of's'aid second varistor, and a i' t nected to a`terniinal'rofsaid seriesfp/ath, nl .leakage path.`Y

Y including a iXed Yresistance .elementhaving its'fterrninalsiYVconnected respectively"directlytozsidigridandfcathocle,.jY

' atvl'ristor through `whichcurrent;frcvrrisaid` souceatiowsf Y Itocontrcl-thepotential drop'between said;coinnri'onA terrnifY nal ofsaid condenser and series path and said cathode dependent upon theaverage illumination of said cell, and a load circuit connected betweensaid anode and cathode.

l5. An electro-optical system for determining the percentage modulationofthe light from a source of mod-ulated light comprising a lightsensitive electric device adapted to be illuminated by modulated light,means to measure the peak-to-peak voltage of the alternating currentcomponent of an electric current produced by the said illumination ofsaid light sensitive device, and means to measure the average Vvalue ofthe illumination of said device in terms of voltage units correspondingto said peak-to-peak voltage units.

16. An electro-optical system comprising a photoelectric cell, a vacuumtube amplier having a cathode, an anode, and a control electrode, aninput circuit for said amplifier including said cathode and controlelectrode, an output circuit for said amplifier including said cathodeand anode, a coupling circuit for alternating current components onlyconnected between said photoelectn'c cell and said input circuitincluding a varistor, means to change the direct current bias on saidcontrol electrode with respect to said cathode in accordance withchanges in the voltage across said varistor resulting from changes inthe illumination of said photoelectric cell, means to measure thepeakto-peak voltage of the alternating current component in the outputcircuit of said amplifier resulting from the illumination of saidphotoelectric cell, and means to measure the average value of theillumination of said photoelectric cell in terms of voltage unitscorresponding to said peak-to-peak voltage units.

17. An electro-optical system for determining the percentage modulationof the light from a source of modulated light comprising a primaryclosed series circuit including a photoelectric cell, a varistor and asource of direct current, a vacuum tube amplier comprising a cathode, ananode and a control electrode, a high resistance connected between saidcathode and control electrode, a conductive connection from one terminalof said varistor to said cathode, a condenser connected between theother terminal of said varistor and said control electrode, a resistanceconnected in shunt of said condenser, means connected to said anode andcathode to measure the peak-to-peak voltage of the alternating currentcomponent of an electric current resulting from the illumination of saiddevice, and means to measure the average value of the illumination ofsaid device in terms of voltage units corresponding to said peak-to-peakvoltage units.

18. An electro-optical system for determining the percentage modulationof the light from a source of modulated light comprising a primaryclosed series circuit including a photoelectn'c cell, a plurality ofvaristors, a resistor and a source of direct current, said resistorbeing connected to one terminal of one of said varistors and a secondvaristor being connected to the other terminal of said one varistor, avacuum tube amplifier comprising a cathode, an anode and a controlelectrode, a condenser connected from the terminal of said resistor,remote from said one varistor to said control electrode, a highresistance connected between the joined terminals of said two varistorsand said control electrode, a conductive connection from the otherterminal of said second varistor to said cathode, a second resistorconnected in shunt of the terminals of said second varistor, means tomeasure the peak-to-peak voltage of the alternating current component ofan electric current resulting from the illumination of saidphotoelectric cell, and means to measure the average value of theillumination of said photoelectric cell in terms of voltage unitscorresponding to said peakto-peak voltage units.

19. An electro-optical system for determining the percentage modulationof the light from a source of modulated light comprising a primaryclosed series circuit including a photoelectric cell, a plurality ofvaristors, and a source of direct current, one terminal of one of saidvaristors being connected to one terminal of a second of said varistors,a vacuum tube amplilier comprising a cathode, an anode, and a controlelectrode, a condenser connected between the terminal of said onevaristor remote from said second varistor and said control electrode, ahigh resistance connected between the other terminal of said onevaristor and said control electrode, a conductive connection between theterminal of said second varistor remote from said one varistor and saidcathode, a resistor connected in shunt of the terminals of said secondvaristor, means to measure the peak-to-peak voltage of the alternatingcurrent component of an electric current resulting from the illuminationof said photoelectric cell, and means to measure the average value ofthe illumination of said device in terms of voltage units correspondingto said peak-to-peak voltage units.

References Cited in the file of this patent UNITED STATES PATENTS1,313,187 Hull Aug. 12, 1919 1,415,748 Weintraub May 9, 1922 1,650,092Poulsen et al Nov. 22, 1927 1,832,707 Hull Nov. 17, 1931 1,864,670Reynolds June 28, 1932 1,889,758 Nakken Dec. 6, 1932 2,203,882 ShoreJune 11, 1940 2,219,189 Lundstrom Oct. 22, 1940 2,360,233 Hussey Oct.10, 1944 OTHER REFERENCES Article by John Russel, vol. 8, pp. 495-6,Rev. of Scien. Instr. for December 1937.

Thermistors in Electronic Circuits-Electronic Industries, January 1945,pages 76-80. (Copy in Div. 54.)

